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When DNA Changes – Chap. 17. http://www.rottentomatoes.com/m/xmen/trailers.php.
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When DNA Changes – Chap. 17 http://www.rottentomatoes.com/m/xmen/trailers.php Left: A normal fruit fly (drosophila). Middle: A fruit fly with its legs jutting from its head; a mutation induced by radiation. Right: A disastrous effect of mutations on the human body. The boy is a Chernobyl nuclear plant accident victim.
DNA is stable • bases (A,T,C,G) are protected inside of sugar-phosphate backbone • tight wrapping around proteins called histones (eukaryotes) and coiling do not allow for direct access to DNA • complimentary base pairing to fix any accidental “changes”
DNA is not stable • mutations can be beneficial, neutral, or harmful for the organism • mutations are random • there are lots of places for mutations to occur; however, not all mutations matter for evolution • somatic mutations occur in non-reproductive cells and won’t be passed onto offspring
only mutations that matter to large-scale evolution are those that can be passed on to offspring • these occur in reproductive cells like eggs and sperm and are called germline mutations
chromosomal defects are surprisingly frequent events • about 20% of all human conceptions have them • many, if not most, irregular chromosomes are the result of a failure of meiosis in the production of sperm and ova (egg) • most are so severe as to cause spontaneous abortions early in pregnancies • mutations in the DNA molecules that make up chromosomes are not usually detected
Frequency of Mutations • in humans and other mammals, mutations occur at the rate of about 1/50 million nucleotides added to the chain • with 3 billion base pairs in a human cell, that means that each new cell contains about 120 new mutations. • however, not that big of a deal because of synonomous codons and “junk DNA” • for eukaryotes this often happens during interphase where all DNA is replicated into two copies before mitosis or meiosis
Damaging DNA mutagens are agents that cause mutations – a failure to repair DNA • radiation—forms of energy (gamma rays, x-rays, UV) that penetrate the cells and damage DNA • chemical—damage DNA through chemical reactions
Three levels of mutational change Level 1. Point or Single-Base Mutations • Involves changes in one or only a few nucleotides in DNA. • 3 Types: I. (Base) Substitutions (Fig. 17.5 in text) – substitutions of a single base pair. Threeoutcomes are... • Silent mutations – has no effect on the structure of the resulting protein since it still codes for the same a.a. because of synonymous codons
Neutral mutations – changes an a.a. in the protein but has no effect its performance • “Drastic” mutations-- has serious effects on future structure and performance of proteins. • Ex) Sickle cell – a codon for an amino acid in hemoglobin mutates from GAA (glutamic acid) to GUA (valine) • Ex) Tay Sacs disease NOVA. One Wrong Letter
II. Chain-Termination Mutations – a new stop codon is produced or a stop codon is mutated into a codon other than a stop III.Frameshift Mutations(Fig. 17.6) result from the deletions and additions of bases into a nucleotide sequence. These cause shifts in the reading of codons during transcription and result in an abnormal protein
Level 2. Chromosomal Mutations • major changes in chromosomal structure rather than simple DNA sequence changes • a complete breaking, misalignment, and rejoining of the double helix in one or more chromosomes
3 Types: • Deletions – entire sections of the chromosome are left out • Inversions – a middle fragment may flip over and then rejoin
Translocations – two nonhomologous • chromosomes break, swap segments, and fuse • together (ex. human #9 and #22 resulting in a type • of lukemia)
Level 3. Transpositions • certain segments of DNA frequently “jump” (sometimes called "jumping genes“) to new locations in the same DNA molecule or a different one • not caused by mutagens but are considered mutagens themselves because they cause the mutation • could an organism “purposely” evolve? • discovered by Barbara Clintock (nobel prize 1983)
